Hermetic seal



HERMETIG SEAL Filed Nov. 50, 1940 L, E. MITCHELL ETAL March 31, 1942.

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INVENTORS LOR/S E. MITCHELL AND ANT/{0N Y J. VASSELLI ATTORNEY PatentedMar. 7 31, 1942 UNITED STATES PATENT OFFICE HERMETIC SEAL Loris E.Mitchell, Bloomfield, and Anthony J.

Vasselli, Newark, N. J., assignors to Radio Corporation of America, acorporation of Delaware Application November 30, 1940, Serial No.367,928

4 Claims. 7 (Cl. 219-40) factory practice that many imperfect welds aremade, apparently because of uneven distribution of current along thelong line of contact. 'There may, for example, be total lack of fusionat one point of the weld, while at another the metal parts may be burnedand splashed'away.

/ tion made according to our invention;

Figure 2 is a sectional detail View of a header made according to theprior art;

Figures 3, 4 and 5 show in succession the steps of our novel method ofpreparing a sheet metal member preparatory to welding;

Figure 6 shows the sheet metal member, prepared according to ourinvention, and the member to which it is to be welded in position forwelding; and

Figure 7 shows in sectioned detail a hermetically tight welded jointaccording to our invention.

The radio tube shown in Figure 1, chosen for illustrating oneapplication of our invention, comprises the usual metal shell I with anoutwardly extending radial flange 2 welded to the flat metal ring 3 ofthe header 4. The metal ring 3 is hermetically joined to a glass button5 in which is sealed the lead-in conductor for the electrodes 6 of thetube, and preferably the ring is joined to the glass button through anintegral metal annulus, U-shaped in cross section,

to isolate the glass-to-metal seal from the thermal andmechanical shockcaused by welding the shell flange and header ring. The usual headerring is embossed, as shown in Figure 2, with an, annular bead 1 midwaybetween the peripheries of the ring and concentric with the ring. Thebead is pressed into the ring by embossing tools which draw the metalupward from the face 0? the ring. The thickness of the metal in thebeac; is thinned by the drawing operation and it is probable the metalof the bead is considerably weakened so that pressure applied to thebead by welding electrodes collapses the bead, reduces the contactwelding resistance and produces imperfect welds.

The header made according to our invention is provided with a flat ringlarger in diameter than the conventional ring and the outer edge of thering is folded inwardly and back upon the ring, and a thickened bead isformed along the inner edge of the folded portion. As shown in detail inFigures 3, 4 and 5 the outer portion 8 of the ring 3 is bent upwardly,first to an upstanding position and then to an inwardly inclinedposition about as shown in Figure 3. While the ring is held in thebacking die 9 and marginal die l0, annular beading tool II with anannular groove in its face descends upon the upper edge of the foldedportion 8 and presses the folded portion against the upper face of thering. The upset metal of the folded portion readily flows and fills therecess in the beading tool and forms a solid embossed bead l2, of thecoin type, on the ring.

The header ring with our novel bead is then placed between the annularwelding electrodes I3 and H which press the shell flange against thebead with considerable force and welding current from source I5 ispassed through the shell flange and header ring to melt the bead andWeld the header and shell together. Good results have been obtained inmaking uniform.

hermetic header-to-shell welds in radio tubes having a shell diameter ofabout one inch and a shell and header wall thickness of .016 to .020inch, the welding being made with an electrode pressure of more than1400 pounds and a welding current of about 60,000 amperes flowing fortwo to three cycles of a commercial power source of cycles per secondalternating current.

It is our belief that the uniform hermetic welds may be attributed tothe solid bead l 2. The high amplitude shortetime Welding current andhigh mechanical pressure, necessary to fuse and weld the parts beforeoxidation of the parts'sets in, require a bead that will not collapse atthe beginning of the heating and hence permit short circuiting of thewelding electrodes, but will support the electrodes and permit gradualheating and welding. Many of the failures with the com ventional typebead of Figure 2 were apparently caused by premature Iiquefaction of thehollow and relatively light bead, and arcing between the relatively coolshell flange and pool of molten metal at the head. This result ismanifested by excessive splashing of molten metal from the weldedjunction and imperfect hermetic seals.

We believe, further, the better welds are made according to ourinvention because of improved electrical and heating conditions of theweld region. The length of the current path through the welded pieces isincreased. The current flows from the head around the bend in the headerring and to the other electrode with negligible current flowing instraight lines between the electrodes because of the metal oxides andother insulating impurities between the surfaces of the header ring andits folded bead. The increase in the resistance of the current paththrough the header decreases the proportional value of the contactresistance between the bead and the shell flange, thus minimizing theeffect of variable contact resistance caused by random scale and oxideson the bead. Further, heat loss caused by conduction from the heatedregion at the bead during welding to the lower electrode issubstantially reduced by the additional layer of pensive in manufacturebecause of the simple dies used in forming the bead. A solid bead ismade with relatively low die pressures compared to the die pressuresnecessary for raising a bead from solid metal, by the conventional coinembossing aemsvi method. Ciur invention provides inexpensive hermeticseals along an extended line of contact between two sheet metal parts.

V le claim:

1. A radio tube envelope comprising a sheet metal sheli with anoutwardly extending flat radial flange at the rim of the shell, meansfor closing the end of the shell comprising a header with a sheet metalflat ring coextensive with said flange, the peripheral portion of thering being folded back upon itself and the edge of said peripheralportion being formed into a bead substantially thicker than the sheetmetal, said bead being resistance welded to said flange.

2. The method of fabricating a metal envelope having a shell flanged atits rim and a header ring hermetically joined to the flange comprisingfolding the outer edge portion of the ring inwardly and back upon thering, forming a thickened bead along the inner edge of the foldedportion, and, while pressing the bead against the shell flange, passingwelding current between the bead and flange.

The method of joining two sheet metal members along an extended line ofcontact comprising bending an upstanding flange in the edge of onemember, upsetting said flange against the side of said one member toform a bead, pressing said bead against the side of the other of saidmembers and passing welding current between said bead and the other ofsaid members.

e. The method of joining two sheet metal pieces along an extended lineof contact comprising folding the edge portion of one piece back uponitself, forming a bead in the folded portion substantially thicker thanthe sheet metal of said one piece, pressing said head against the otherpiece and passing welding current between the two pieces through saidbead.

LORIS E. MITCHELL. ANTHONY J. VASSELLI.

